1. Field of the Invention
The present invention relates to transmission of electromagnetic energy between coaxial transmission lines of differing impedances. More particularly, the invention relates to an input transformer of a bandpass filter that is continuously variable to optimize power transfer from a radio-frequency (RF) source into a resonant cavity of a klystron.
2. Description of Related Art
In microwave amplification devices, such as klystrons, it is often necessary to couple an RF signal either into or out of a resonant cavity. An RF input signal inductively coupled into the resonant cavity can be used to velocity modulate an electron beam traveling through the cavity. The velocity modulated beam then induces a current into a subsequent resonant cavity having RF power that is substantially greater than the power of the input signal. A high power RF output signal can then be removed from the device and put to use.
It is desirable to avoid any unnecessary energy loss of the RF signal in order to obtain maximum efficiency from the microwave amplification device. Typically, transmission lines are used to convey the RF signal to and from the resonant cavities. All transmission lines have a characteristic impedance that is dependent upon the geometry and material properties of the transmission line. If two transmission lines of different characteristic impedance are directly joined, some of the energy travelling along either line will reflect at the interface of the two lines, preventing total energy transfer from one transmission line to the other. Thus, to obtain maximum efficiency from the microwave amplification device, the impedance of the resonant cavity must be matched to the impedance of the input transmission line.
It is well known in the art to provide a transformer to improve energy transfer between the cavity and the RF signal input. One type of transformer, known as a quarter wavelength transformer, is frequently used because it permits total energy transmission while occupying relatively little space. Quarter wavelength transformers allow complete energy transfer between the transmission lines only if the impedance (Z.sub.0) of the transformer is equal to the geometric mean of the impedances of the two transmission lines that the transformer connects. The position of the transformer along a transmission line relates to the phase characteristic of an RF signal conducted on the transmission lines, and the impedance of the transformer relates to the amplitude characteristic of the RF signal. The adjustability of these two variables (position and impedance) effects the overall performance of the entire transmission line system.
The transmission line system into the resonant cavity of the microwave amplifier can be characterized as a bandpass filter that defines the characteristics of the RF input signal. The bandpass filter is terminated by the electron beam impedance (typically 12,000 ohms). The filter consists of the inductance of the resonant cavity, the inductance of the coupling of the transmission line into the cavity, and the impedance and capacitance of the transmission line system (including the transformer) that carries the RF input signal. Thus, variation of the position and impedance of the input transformer alters the bandpass filter characteristics.
Conventional input transformers allow only incremental (step) adjustment to position and impedance. Each time an adjustment is made, the transformer must be disassembled and/or replaced. This technique is time consuming, expensive, and unlikely to produce an input transformer or bandpass filter with optimum characteristics. Moreover, the transformer cannot be continuously adjusted to compensate for real-time changes to the microwave amplification system and input RF signal.
Therefore, a need presently exists for an improved coaxial quarter wave input transformer and bandpass filter that can vary both phase and amplitude of an input RF signal across a continuum of values without requiring disassembly of the transmission line system.